Williams was a slugger without a position; he had tremendous power but was inconsistent. He had a tremendous season in the Braves' system in 1969, hitting .340 with 33 homers (leading the league) and 107 RBI for the Greenwood Braves of the Western Carolinas League; that season would eventually earn him election to the South Atlantic League Hall of Fame in 2004 (the circuit had been renamed in intervening years). In 1970, he continued to rake, collecting 24 homers, 78 RBI and a .309 batting average between the AA Shreveport Braves and AAA Richmond Braves. He made his major league debut in September of 1970, going 7 for 19 (.368) in 10 games. In his rookie year, he split time between catcher, third base and first base, displaying defensive prowess at none of the three spots. In the minors, he had played mainly first base, and did not have a single game of experience behind the plate before his team made him try the position for the first time in the major leagues in 1971. The Braves attempted to turn him into a full-time catcher in 1972, to take advantage of a very strong arm, but with mixed results. He did manage 116 games behind the plate that year, and continued to hit well (.257 with 28 homers and 87 RBI), but his handling of pitchers left a lot to be desired.

He did fit the mold of players that Baltimore Orioles manager Earl Weaver liked, as his power and ability to draw walks fit right into the team's offensive blueprint. Weaver insisted on acquiring Williams for the 1973 season. The Orioles gave up a veritable king's ransom to acquire him: second baseman Davey Johnson, pitchers Pat Dobson and Roric Harrison and catcher Johnny Oates for Williams and minor leaguer Taylor Duncan. He should probably have been turned into a full-time designated hitter at that point and left to prosper, but it was the first year the new rule was in effect, and teams had the idea that the new position was made for an aging player, not a 24-year-old budding star like Earl. The O's used veteran Tommy Davis as their primary DH from 1973 to 1975, even though he was not that fearsome a hitter anymore, and it meant that Williams had to play the field. He played 95 games as a catcher and 42 as a first baseman in 1973, hitting .237 with 22 homers and 83 RBI, then with a similar split in 1974 hit .254 with 14 homers and 52 RBI in 118 games. The Orioles could live with his poor defense, since they won the AL East title both years, although they were ousted by the Oakland Athletics in the American League Championship Series both seasons. Williams hit .278 with two doubles and a homer in the 1973 ALCS, but was only 0 for 6 in the 1974 ALCS, having by then lost his regular starting job. It was clear that the stress of playing catcher regularly was having a detrimental effect on Williams' hitting as well. Thomas Boswell once said that Earl never could fully master the concept that catchers had to change the sequence of signs with a runner on second base, and he would regularly get confused, to the extent that you could see figurative smoke coming out of his ears as he attempted to work out what signs he should now flash. The Orioles gave up on their experiment after two years; they thus acquired two veterans, Dave Duncan to be their catcher and Lee May to play first base in 1975.

Williams was left hanging in the air with those changes, but was only traded on April 17, 1975, after the season had already been underway but before he had played in any game. His destination was back to Atlanta, but this time, in contrast with the first trade, the Orioles received next to nothing in return: pitcher Jimmy Freeman, who would not play in the majors again. In Atlanta, Earl was almost exclusively a first baseman. He played 111 games, hitting .240 with 11 homers ans 50 RBI (an OPS+ of only 82), quite poor production for a first baseman who was supposed to hit in the middle of the line-up. In 1976, he struggled even more with the Braves, hitting .212 in 61 games, when the Braves gave up and sold him to the Montreal Expos on July 24th. The Expos were having a terrible year and used Williams on a regular basis the rest of the way, getting him into another 61 games, during which he hit .237 with 8 homers and 29 RBI in the remainder of the 1976 season. However, he was clearly seen as only a stop-gap measure, because the Expos acquired future Hall of FamerTony Perez from the Cincinnati Reds to play first base, and had another future member of Cooperstown to catch in Gary Carter. With solid major leaguers Mike Jorgensen and Barry Foote backing up the two positions, there was no room for Williams on the team, and the Expos released him on March 28, 1977. A few days later, Williams signed as a free agent with the Oakland Athletics and finally got some regular playing time at DH, although he was also a back-up catcher for 36 games, and got into another 29 games at first base. He played 100 games, hitting .241 with 13 homers and 38 RBI. He was still only 28 at the time, and the A's were heading into a period when they would struggle to find enough major league-caliber players to field a proper team, but they still chose to release Williams in May of 1978, even though he had not played at all that season. Williams headed to the Mexican League and played there for a couple of seasons in 1979 and 1980, before retiring for good.

Williams settled back in his native New Jersey after his playing career. He was diagnosed with acute leukemia in the summer of 2012 and died a few months later, in 2013.